This invention relates to optically uncoupled or nonphase locked semiconductor laser arrays and more particularly to optically uncoupled laser arrays modified to have an overall increased spectral emission bandwidth. An array of closely spaced but optically uncoupled diode lasers has been proposed and developed for electro-optic line modulators and line printers. An example of such a laser array is disclosed in U.S. patent application Ser. No. 808,197 filed Dec. 12, 1985 entitled, "INCOHERENT, OPTICALLY UNCOUPLED LASER ARRAYS FOR ELECTRO-OPTIC LINE MODULATORS AND LINE PRINTERS". In that disclosure, an incoherent diode laser array source is employed as a single solid state light source capable of providing a sheetlike, uniform and high intensity of light. The uncoupled array comprises a series of monolithic index guided diode lasers on a single substrate with their index waveguide cavities formed by layer impurity induced disordering. The essential ingredient is that the individual lasers or emitters of such an laser array operate randomly phased with respect to each other, i.e., they are not sufficiently evanescently coupled to be in phase with each other. If each laser in the array oscillates independently of the other lasers, its optical phase and/or frequency will randomly drift and, as a result, permit optical interference to exist only within the beam of each individual laser rather than among or between the beams of adjacently positioned diode lasers of the array.
Although such laser arrays exhibit the desired single lobe, far field radiation patterns desired for electro-optic line modulators, their optical spectra are narrower than the emission spectra of an LED. The bandwidth or linewidth of these laser arrays are narrower because all the lasers in the array are fabricated to be nominally identical and each laser oscillates in predominately one longitudinal mode relative to their cavities of substantially equal length. Any spectral width in the total emission arises only due to small but unintentional differences existing in the geometrical and optical parameters between the spatial emitters. Also, it is possible that the temporal coherence of these laser arrays will be too large to eliminate optical interference effects which arise from light scattering due to scratches, dust and other foreign micro particles present in the optical system of the modulator or printer.
It is an object of this invention to provide means in the laser array structure to force the individual emitters in the array to lase at different longitudinal modes to thereby increase the overall spectral emission bandwidth of the array laser.
It is another object of this invention to reduce temporal coherence in optically uncoupled index guided array lasers.